Sandpaper grinding machines are used for smoothing, grinding and/or polishing surfaces of soft and/or rigid materials of planks and various metal sheets in field of decoration engineering. Handheld sandpaper grinding machines includes manual type and electrical type. Currently, grinding sheets, such as sandpaper, are generally used for manually grinding or polishing working surfaces, such as surfaces of planks. During the course of manually grinding, a user holds sandpaper in his hand and moves the same on working surfaces backwards and forwards. However, the grinded surfaces would still not be smooth enough. Thus, manual grinding is a very laborious task. To make it easy to grind manually, sandpaper is usually placed on and held by a sanding block (also called as sandpaper block), to make it convenient for the user to grasp. Grinding blocks are well-known in the prior art. In most paint engineering for decorating walls, workers carry out polishing procedures by holding sandpaper by hand or placing sandpaper on surface of sponges. In this matter, the working intensity of the polishing procedures is heavy, the working efficiency is low, but the polished surface is not smooth enough. Currently, there are several kinds of conventional handheld electrical grinding machines, which are suitable for grinding rigid materials, such as metal. But, they are not suitable for polishing procedures for plaster walls since their speed of revolution is very high, and it is very hard to hold such kinds of machines during its operation. In this connection, such kinds of grinding machines have not been applied in polishing plaster walls. There are also several kinds of small-sized electrical grinding machines which make it possible for holding the machines during operation. However, as the power of the machines is low, they are not suitable for industrial use.